Miniaturized cartridge fuse for small current intensities having large time-lag



Jan. 28, 1969 I F. J. KOZACKA 3,

MINIATU RIZED CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES HAVING LARGE TIME-LAG Filed Sept. 5, 1967 FIG. I

. iNVENTOR= FREDERICK J. KOZACKA BY MWW W United States Patent 3,425,019 MINIATURIZED CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES HAV- ING LARGE TIME-LAG Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed Sept. 5, 1967, Ser. No. 665,626 U.S. Cl. 337-163 2 Claims Int. Cl. H01h 85/06 ABSTRACT OF THE DISCLOSURE A miniaturized static electric time-lag cartridge fuse for small current ratings from a few amperes down to fractions of an ampere, capable of withstanding high inrush currents without blowing. The term static is used in this context with reference to fuses not involving separable contacts for interrupting relatively small overload currents of excessive duration, and the term miniaturized is used in this context to refer to fuses having casings of smaller sizes than National Electric Code Fuses having the same current rating and voltage rating.

Background of invention In static time-lag fuses the fusible element has an overlay of a metal having a lower fusing point than the fusible element. The overlay absorbs heat and it initiates when it fuses a metallurgical reaction resulting in an interruption of the current path formed by the fusible element. Non-static time-lag fuses include a pair of contacts which are bonded together by a solder joint, and of which one is under the action of a biasing spring tending to separate the contacts, and thus to interrupt the circuit in which the contacts are included. This action can only take place after the solder joint has softened as a result of the flow of a small overload current of inadmissible duration.

It has not been possible heretofore to make static timelag fuses for small current intensities exhibiting long timelags before blowing.

Normally fusible elements are made of copper, or silver, particularly if it is desired to achieve a high interrupting capacity and a current-limiting action on occurrence of mayor fault currents, or short-circuit currents. Fusible elements of copper, or silver, have a high conductivity. If given such a small cross-sectional area as required for very small current ratings, erg. amp'., fusible elements or wires of copper or silver become so difiicult to handle, and have so little dimensional stability, as to preclude their use in time-lag fuses. One may consider substitution of a metal having a much smaller conductivity than copper and silver for copper and silver, thus obtaining a fusible element which combines a relatively low current-carrying capacity, and a sufiiciently high mechanical strength. However, conventional high resistivity metals do not lend themselves to be quickly severed by a metallurgical reaction incident to melting of a relatively low melting point overlay metal which is placed thereon. Thus the requirement of high resistivity and sufliciently high mechanical strength, on the one hand, and the requirement of relatively rapid severance of a current-path by means of an overlay having a relatively low fusing point, on the other hand, are generally mutually exclusive.

Summary of invention I have discovered that a fusible wire of an alloy of copper and tin, in particular phosphor bronze having a copper content of about 95% and a tin content of about 5% can readily be severed by an overlay of a low melting point metal such as, for instance, tin. Such alloys have a relatively high resistivity, and wires formed by such alloys lend themselves to be used as fusible elements for small time-lag fuses having small current ratings from a few amps down to frictions of one amp. However, fuses including such wires and an appropriate overlay for severing the wires by a metallurgical reaction on the occurrence of protracted ove'rloads have much too small time-lags to withstand high in-rush currents such as those caused by starting small electric motors.

Some improvement can be achieved in regard to timelag by forming the wire into a loop including parallel sections which mutually heat each other. This raises the problem of electrically insulating the parallel sections of the loop. The loop configuration increases the time-lag, but not sufiicient to preclude undesired blowing of the fuse on relatively high inrush currents. The electrical insulation of serially connected parallel sections of a looped fusible element by a pulverulent arc-quenching filler results in an intolerable decrease of the time-lag. I have found that electrical insulation of the sections of a loop-shaped wire and suffieient time-lag can be achieved by mounting a looped-shaped tubular sleeve of woven glass fibers on the looped wire of high copper and small tin content phosphor bronze, or like alloy. Such a sleeve performs two functions, i.e. it isolates electrically the constituent loops of the fusible wire and it de-ionizes and cools the products of arcing resulting from the vaporization of the fusible wire. In such an arrangement it is further of critical importance, in order to achieve the required time-lags, to thermally isolate the looped wire and the looped sleeve of woven glass fibers from the inner surface of the casing. Such thermal insulation can and must be achieved by establishing a void between these parts rather than by interposing a solid thermal insulator therebetween. This is so because the presence of such a void is far more effective in limiting heat exchange between the hottest point of the fusible wire, the wiresevering low-melting-point overlay, and the fuse casing.

Miniaturized low-current-carrying-capacity time-lag fuses are generally required not to blow within 6 sec. on inrush currents of three times their current rating, and to have longer blowing times if the inrush current is less than three times the rated current. Such time-lags are achieved in fuses embodying this invention by the cumulative action of a fusible element in Wire form of a high resistivity alloy including copper and tin, the loop configuration of the wire, the electrical insulation of the portions of the loop-shaped wire without resorting to a solid or pulverulent arc-quenching filler by means of a woven glass-sleeve, the presence of a solidified drop of a metal having a substantially lower melting point than said high resistivity alloy capable, on reaching a predetermined temperature, of severing said wire by metal interdiifusion, and the highly effective thermal insulation from the casing of the fuse of the fusible wire and the wire-severing solidified drop of a low melting point metal by means of a void or gaseus body separating these parts from the inner surface of the casing of the fuse.

Prior art Miniaturized fuses having time-lags of the above order and capable of interrupting short-circuit currents have todate only been achieved by much more complex structures than those embodying this invention. U.S. Patent 2,832,868 to Frederick J. Kozacka, Fillerless One-time National Electrical Code Fuse, Apr. 29, 1958 and U.S. Patent 2,833,890 to Philip C. Jacobs, J-r., Fillerless One Time Fuses, May 6, 1958 disclose what is believed to be the closest prior art, but have no bearing on the instant invention inasmuch as these patents do not refer to miniaturized low current-carrying-capacity time-lag fuses capable of achieving the above referred-to time-lag characteristics.

Brief description of drawings FIG. 1 is a longitudinal section of a fuse embodying the present invention;

FIG. 2 is a section along II-II of FIG. 1;

FIG. 3 is a view of the fusible element of the structure of FIGS. 1 and 2 before being enclosed in a sleeve of glass cloth and before being folded to form a pair of loops; and

FIG. 4 is a view of the fusible element of the structure of FIGS. 1 and 2 after being inserted into a sleeve of galss cloth and before being folded to form a pair of loops.

Description of prefer redl embodiment Referring now to the drawings, and more particularly to FIGS. 1 and 2 thereof, reference numeral 1 has been applied to indicate a tubular casing of an electric insulating material. Casing 1 is closed at its ends by a pair of electroconductive terminal elements or ferrules 2. Each ferrule 2 defines at the outer end surface thereof a recess which is filled with a pool of solidified solder 3. Reference numeral 4 has been applied to indicate a fusible element in wire form of a high resistivity alloy including copper and tin. Wire 4 is preferably a Phosphor bronze having a copper content of about 95%, a tin content of about 5% with an addition of less than 1% phosphorus having an electrical conductivity in percent IACS at 68 deg. F. of about 16% e.g. Riverside Alloy 30" of the Riverside Alloy :Metal Division, H. K. Porter Company, Inc. Fusible wire 4 includes two radially outer portions 4a and a radially inner portion 4b integral with the radially outer portions 4a. The radially outer portions 4a overlap the radially inner portion 4b and both portions, 411, 4b form a double loop and are connected in series. Each radially outer portion 4a has an axially outer end connected to one of said pair of terminal elements or ferrules 4. To this end the axially outer ends of the radially outer portions 4a of wire 4 project transversely through terminal elements or ferrules 4 into the pools of solder 3. The radially outer portions 4a of wire 4 operate as heat dams for the radially inner portion 4a thereof, i.e. they limit heat flow by conduction from the radially inner portion 4b of wire 4to the terminal caps or ferrules 2. In addition thereto the radially outer portions 4a of wire 4 perate as heaters for heating the radially inner portion 4b of wire 4.

Reference numeral has been applied to indicate a solidified drop of a metal on wire portion 4b having a substantially lower melting point than the high resistivity alloy of which wire 4 is made and capable, on reaching a predetermined temperature, of severing wire 4 by a metallurgical reaction in the nature of a metal interdilfusion process. To be more specific, metal drop 5 may be of tin and result in a diffusion of the copper-phase of wire 4 into it when drop 5 melts. Rates of diffusion increase with increasing temperature and, as the temperature increases, some of the tin drop 5 diffuses into the bronze of which wire 4 is made, increasing its resistivity, and ultimately causing interruption of the current path formed by wire 4.

A tubular sleeve 6 of woven glass fibers is mounted on wire 4 and surrounds the outer surfaces of its radially inner portion 4b and of its radially outer portions 4a. The space between tubular sleeve 6 and the inner surface 1a of easing 1 is free from any solid arc-quenching filler, and forms a gaseous thermal insulator establishing a relatively steep temperature gradient between wire 4 and tubular sleeve 6, on the one hand, and the inner surface 1a of easing 1, on the other hand. As mentioned above, sleeve 6 performs the dual function of electrically insulating the radially outer portion 4a and the radially inner portion 4b of wire 4 from each other, and of condensing and deionizing the metal vapors resulting from vaporization of wire 4 following formation of a break therein and kindling of an are at the point of break-formation.

The time-current curve of a fuse embodying this invention is, in essence, the fast time-current curve of any fuse having a fusible element in wire form, but exhibits considerable time-lag in the range of small multiples of the rated current, e.g. 6 seconds at three times the rated current. The table below refers to a fuse embodying this invention, and having the geometrical configuration Of FIGS. 1 and 2, and having a current rating of 3 amps.

Current in amps: Fusing time in secs.

It will be understood that I have illustrated and described a preferred embodiment of the invention, and that various alterations may be made in the details thereof without departing from the spirit of the invention as defined in the appended claims.

I claim:

1. An electric time-lag fuse for low current intensities comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a fusible element in wire form of a high resistivity alloy including copper and tin conductively interconnecting said pair of terminal elements, said wire including a radially inner portion and said wire further including two radially outer portions integral with said radially inner portion each overlapping said radially inner portion, each being serially connected with said radially inner portion and each having an axially outer end connected to one of said pair of terminal elements;

(d) a solidified drop of a metal having a substantially lower melting point than said alloy and capable on reaching a predetermined temperature of severing said wire by metal interdilfusion supported by said radially inner portion of said wire adjacent the center of said radially inner portion;

(e) a tubular sleeve of woven glass fibers mounted on said wire and surrounding the surfaces of said radially inner portion thereof and of said two radially outer portions thereof; and

(f) the space between said tubular sleeve and the inner surface of said casing being free from any solid arcquenching filler and forming a gaseous thermal insulator establishing a relatively steep temperature gradient between said wire, said tubular sleeve, and the inner surface of said casing.

2. An electric fuse as specified in claim 1 wherein said fusible wire is of Phosphor bronze having a copper content of about and a tin content of about 5% and an electrical conductivity in percent IACS at 68 deg. F. of about 16% and wherein said solidified drop of a metal having a lower fusing point than said alloy consists substantially of tin.

References Cited UNITED STATES PATENTS 2,337,938 12/1943 Serfling et a1 337-158 X 2,856,488 10/1958 Kozacka 337-159 X 3,261,950 7/1966 Kozacka 337-158 X BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

US. Cl. X.R. 337-158, 222 

